Photo-application is a general term of technology to manufacture various precision parts by a process comprising applying a radiation-sensitive resin composition to a surface of a workpiece, then patterning the coating film by photolithography and performing electroforming techniques, such as chemical etching, electrolytic etching and electroplating, singly or in combination, using the patterned film as a mask. This photo-application is the main stream of precision fine work technology at present.
In recent years, with downsizing of electronic instruments such as cellular phones, high integration and multilayer metallization of LSI have been rapidly advanced, and application of a multipin mounting method to boards (printed wiring boards, referred to as “board(s)” simply hereinafter) that are used for mounting LSI on electronic instruments has been desired. For example, bear chip mounting by TAB system or flip chip system has been paid attention. In such a multipin mounting method, a protruded electrode called a bump that is a connecting terminal needs to be arranged on a LSI chip with high precision, and it is thought that formation of a bump with much higher precision will be further required in the future to cope with further downsizing of LSI chips.
There are bumps of various shapes, such as ball bump, mushroom bump and straight bump, and in the conventional bumps, the height is mainly not less than 15 μm, more specifically about 20 to 30 μm.
With respect to a material to form a bump of such a height, the present inventors have already proposed a radiation-sensitive resin composition that is preferable as a bump-forming or wiring-forming material improved in adhesion to a substrate in the development stage of photolithography, wettability with a plating solution and plating solution resistance (Japanese Patent Laid-Open Publication No. 39709/2000). If this radiation-sensitive resin composition is used in the photo-application, a bump-forming material that exhibits excellent developability with an alkali developing solution, sufficient resolution in a film thickness of not less than 20 μm, adhesion to a substrate in the development stage, wettability with a plating solution and plating solution resistance can be obtained, and hence, formation of a bump having a height of about 20 to 30 μm with high precision becomes feasible.
By the way, when a LSI chip and a board are connected by a bump as described above, a stress is brought about by a difference in a coefficient of thermal linear expansion between the LSI chip and the board, and they are sometimes disconnected. To inhibit such connection failure and to enhance reliability of an element, it is desirable to increase the amount of gold or solder of the bump that is a connecting part to thereby relax the stress. In this case, if a distance (also referred to as a “pitch” hereinafter) between bumps is relatively wide as in the conventional LSI chips, an area of the gold or solder portion can be increased to assure the amount of gold or solder. However, if the pitch is narrowed because of higher precision of bumps, the area of the gold or solder portion cannot be increased, and therefore, the height of the bump needs to be increased to assure the amount of gold or solder.
However, if an attempt to form a bump of a greater height (also referred to as a “high bump” hereinafter) on a LSI chip (also referred to as a “chip” simply hereinafter) with high precision by means of photo-application is made using a material comprising a radiation-sensitive resin composition that is used in the usual bump formation, the following problem is brought about.
That is to say, it is difficult to form, on a chip substrate, a coating film having a film thickness necessary for forming a high bump, and even if a coating film of such a film thickness can be formed, the film has a low transmittance. Therefore, when the film is irradiated with a radiation, the surface of the film is cured but the portion near the chip substrate is not sufficiently cured, and consequently, a desired pattern cannot be obtained with high precision.